This application is based on and incorporates herein by reference Japanese Patent Application No. 11-344689 filed Dec. 3, 1999.
The present invention relates to a GPS (global positioning system) receiver that extracts GPS data from a GPS satellite signal of intermediate frequency by scanning signal-to-noise ratios of the GPS signal of the intermediate frequency over search frequency band and detecting a frequency at which the signal-to-noise ratio is maximum above a threshold. The present invention further relates to a terminal device using the GPS receiver.
GPS receivers and portable terminal devices using the GPS receivers are used for telecommunication. Each GPS receiver 1 is constructed typically, as shown in FIG. 7 in a block diagram form, with a clock unit 2 including a clock generator, a radio frequency (RF) unit 3, a data extraction unit 4, a data memory unit 5, a data calculation unit 6 and an antenna 7. The clock unit 2 generates a reference clock signal at a fixed frequency so that the RF unit 3, data extraction unit 4 and data calculation unit 6 operate in synchronism with the reference clock signal.
Specifically, the RF unit 3 mixes a GPS signal of radio frequency received from a GPS satellite (not shown) by the antenna 7 with its local oscillation signal to generate a GPS signal of intermediate frequency. The data extraction unit 4 sets, as shown in FIG. 8, its center frequency f1 which deviates from a reference frequency f0 by a frequency corresponding to the estimated Doppler shift of the GPS satellite. It also sets a search frequency band fW1 based on the center frequency f1 and a maximum change (error) in frequency of the clock signal which will be caused due to changes in temperature of the clock generator.
The data extraction unit 4 scans signal-to-noise ratios (SNR) of the GPS signal received from the RF unit 3 over the search frequency band fW1, and detects a frequency which provides a maximum signal-to-noise ratio above a predetermined threshold signal-to-noise ratio Th. Thus, the data extraction unit 4 extracts GPS data from the GPS signal of the intermediate frequency. The extracted data are temporarily stored in a data memory unit 5 and used in the data calculation unit 6. The above operation of the data extraction unit 4 is controlled by control commands from the data calculation unit 6.
It is to be noted that the frequency of the clock signal of the clock unit 2 also changes due to aging of the clock generator. As the change or error in the frequency of the clock signal (aging error) increases, the frequency band which provides the maximum signal-to-noise ratio above the threshold Th of the GPS signal generated by the RF unit 3 deviates from the search frequency band fW1. In case that the frequency f2 which provides the maximum signal-to-noise ratio greatly deviates from the search frequency band fW1 as shown in FIG. 9, the frequency of the maximum signal-to-noise ratio cannot be detected resulting in failure of extracting GPS data appropriately.
If the search frequency band is set in correspondence with a maximum aging error, the search frequency band will have to be set wide. As a result, the data extraction unit 4 requires a longer period to scan the signal-to-noise ratios over the widened search frequency band. The GPS receiver 1 requires more electric power contrary to a requirement that a terminal device using the GPS receiver consumes less power.
It is therefore an object of the present invention to provide a GPS receiver and a portable terminal device using the GPS receiver, which can extract GPS data accurately from a GPS signal of intermediate frequency even when the frequency of a reference clock signal changes.
According to the present invention, a GPS receiver comprises a clock unit, a radio frequency unit, and a data extraction unit. The data extraction unit scans signal-to-noise ratios of a GPS signal generated by the radio frequency unit over a search frequency band and detects a frequency at which a signal-to-noise ratio of the GPS signal is higher than a threshold ratio in the searched frequency band to extract GPS data from the GPS signal. The data extraction unit sets the search frequency band to a first frequency band in a normal mode and to a second frequency band in a band correction mode. The first frequency band has a first center frequency determined based on an estimated Doppler shift of a GPS satellite and a band width determined based on a generally maximum temperature-dependent change in a frequency of the reference clock signal. The second frequency band has a band width determined based on a generally maximum aging-dependent change in the frequency of the reference clock signal.
Further, the data extraction unit further sets the search frequency band to a third frequency band after the band correction mode. The third frequency band has a center frequency around a second center frequency which provides a generally maximum detected signal-to-noise ratio above the threshold as a result of scanning the signal-to-noise ratios over the second search frequency band in the band correction mode. The third search frequency band has the same band width as that of the first search frequency band, or a band width narrower than that of the first search frequency band.
The mode switching from the normal mode to the band correction mode may be made manually by a user, or automatically based on the signal-to-noise ratios scanned in the normal mode or a period of use of the clock generator.